Where's the best place to buy vitamins?

I'm hypothyroid and have had a few blood tests which show I could benefit from supplementing a few vitamins.My b12, vit d and ferritin are all low in range and I'd like to add a vit c and b complex for good measure.I've bought some in the supermarket but figure there must be a better quality product out there. The dosages vary so much from one to the next.Any recommendations or suggestions would be greatly appreciated.